a. Field of Invention
The invention concerns a background field or base net for image generating (imaging) devices using nuclear spin resonance technology (such as nuclear spin tomography) and more particularly to a device which contains at least six annular super-conductive single (individual) coils arranged on a tubular coil support along a common axis placed behind each other with which, within an image region, a background magnetic field of a first predetermined field strength and sufficient homogeneity is generated. These single coils have predetermined ampere turns and are arranged in pairs at a equal distances with respect to a plane of symmetry which runs through the center of the image region perpendicular to the common axis. More particularly a pair of inner, middle, and outer single coils formed, spaced at increasing distances from the plane of symmetry.
b. Description of the Prior Art
A field magnet similar to the one described above is found in U.S. Pat. No. 4,385,277.
In the area of medical diagnosis, image generating processes have been developed in which integral resonance signals of nuclei of a given element of a body, particularly a human body or body part are analyzed by actual measurement, or by computation. From the spatial spin density and/or relaxation time distribution a image can be constructed similar to a x-ray-tomogram. The corresponding processes are known as "nuclear spin tomography" (nuclear magnetic resonance tomography) or "zeugmatography".
For nuclear spin tomography, a strong magnetic field generated by a so-called background field magnet which, within an image or examination region of predetermined extent, should be as homogeneous as possible. The body to be examined is introduced within said region along an axis which is general coincides with the orientation axis of the magnetic field. This field is superimposed by stationary and/or pulsed gradient fields. For the excitation of the individual atomic nuclei in the body into nuclear precession, a special antenna device is required in addition with which, temporarily, a high frequency magnetic alternating field can be produced. Frequently, the antenna is also used to receive the HF-signals to be regenerated if no special measuring coils are provided for that purpose.
The background field magnet can, in particular, consist of six annular superconductive single coils aligned in the direction of the magnet field orientation. The coils are arranged adjacent to each other, on a common coil support. The support has grooves for holding the coils as described in U.S. Pat. No. 4,385,277 mentioned above. These grooves are arranged in pairs symmetrically and in predetermined median distances from a plane of symmetry which runs perpendicularly to the orientation axis. The windings of the single coils are placed into the grooves. The individual distances of the grooves from the plane of symmetry, the number of the conductor windings of the single coils to be inserted, and the maximal current flowing through coils are determined from the desired field strength of the magnet in a known manner (see "Journal of Applied Physics", Vol 38, No. 6, May 1967, pages 2563 to 2586). Thus, for a given background field magnet with a coil support having known dimensions for a desired field strength, the individual parameters mentioned above are specifically defined. The field magnet described above is, in addition, fitted with a number of additional coils. These additional coils are electrically isolated from and are excited independently of the first six single coils.
For the nuclear spin tomography super-conductive background field magnets are employed of the same dimensions but intended for different operating field strengths corresponding to magnetic inductions of, for example 0.5 T to 2T. Since the superconductive material for the single coils for this purpose is very expensive and has a physically and technically predetermined maximum current carrying capacity, background field magnets of different strength require different cross sections for their windings, i.e., these single magnets have to be manufactured with different electric ampere turns. Since, furthermore, for nuclear spin tomography, the background magnetic fields in the effective image region have to be extremely homogeneous, the spatial position of each single coil has to be aligned for each designed field strength. In addition, the cross sectional area of winding and its ampere turns must also be changed. This, however, generally necessitates a new coil support with different dimensions fashioned for each desired field strength. The consequences are costly, and storage space as well as long lead times from ordering to delivery of the magnets are required.